At least, that's the file path for me. I installed it to the default location, so that's probably where yours is too.

I might as well just tell you the rest. Go into the folder, (C:\Program Files\SEGA\Medieval II Total War\) and just drag the other exe into it. It'll say "blah blah blah already a file named that would you like to replace it blah blah?" and you click yes. First, you should probably make a copy of the real exe (the one that's in the folder I mentioned) and save it to a different place, like your desktop or my documents so you don't have to reinstall the game if this doesn't work. Good luck!